Enthalpy, calorimetry and Hess’s law · Grade 10
An enthalpy change, ΔH, is the heat energy transferred in a reaction at constant pressure. It is negative for exothermic reactions and positive for endothermic ones. It can be measured in a calorimeter from a temperature change, or worked out from other enthalpy changes using Hess’s law.
Key facts
- Heat taken in by water or a solution: q = mcΔT, with m in g, c = 4.18 J g⁻¹ K⁻¹ for water and ΔT in K. Then ΔH = −q ÷ (moles that reacted), in kJ/mol.
- Standard conditions: 298 K, 100 kPa, solutions 1 mol dm⁻³ and every substance in its standard state. ΔH° means an enthalpy change under standard conditions.
- Standard enthalpy of formation, ΔHf°: the enthalpy change when one mole of a compound forms from its elements in their standard states. For an element in its standard state ΔHf° = 0.
- Standard enthalpy of combustion, ΔHc°: the enthalpy change when one mole of a substance burns completely in oxygen.
- Hess’s law: the enthalpy change of a reaction is the same whatever route is taken, because energy is conserved.
- From formation data: ΔH = ΣΔHf(products) − ΣΔHf(reactants). From combustion data: ΔH = ΣΔHc(reactants) − ΣΔHc(products).
- Values from simple calorimetry are usually less exothermic than data-book values: heat is lost to the surroundings and some fuel burns incompletely.